Conventionally, as a GaN nitride semiconductor, a light-emitting element and a light-receiving element having a multilayer structure based on a GaN layer or an AlGaN layer having a relatively small AlN molar fraction (also referred to as AlN mixed crystal ratio or Al composition ratio) are produced (see, for example, Non-Patent Documents 1 and 2). FIG. 26 illustrates a typical crystal layer structure of a GaN light-emitting diode. The light-emitting diode illustrated in FIG. 26 has a laminated structure in which, after an underlying layer 102 of AlN is formed on a sapphire substrate 101, and then a periodic groove pattern is formed thereon by photolithography and reactive ion etching, an ELO (Epitaxial Lateral Overgrowth)-AlN layer 103 is formed as a template; and an n-type cladding layer 104 of n-type AlGaN having a thickness of 2 μm, an AlGaN/GaN multi-quantum well active layer 105, a p-type AlGaN electron block layer 106 having an Al composition ratio which is larger than that of the multi-quantum well active layer 105 and having a thickness of 20 nm, a p-type cladding layer 107 of p-type AlGaN having a thickness of 50 nm, and a p-type GaN contact layer 108 having a thickness of 20 nm are sequentially stacked on the ELO-AlN template 103. The multi-quantum well active layer 105 has a five-layered structure including a GaN well layer having a film thickness of 2 nm and sandwiched by AlGaN barrier layers having a film thickness of 8 nm. After crystal growth, in order to partially expose a surface of the n-type cladding layer 104, the multi-quantum well active layer 105, the electron block layer 106, the p-type cladding layer 107, and the p-type contact layer 108 thereon are etched off. A p-electrode 109 of Ni/Au is formed on a surface of the p-type contact layer 108, for example, and an n-electrode 110 of Ti/Al/Ti/Au is formed on the surface of the exposed n-type cladding layer 104, for example. By arranging a GaN well layer into an AlGaN well layer, and changing the AlN molar fraction or the thickness of the AlGaN well layer, the emission wavelength is made shorter, or by adding In, the emission wavelength is made longer, so that a light-emitting diode in an ultraviolet region having a wavelength of about 200 nm to 400 nm can be produced. A semiconductor laser can also be produced in a similar manner.
Light emitted from the active layer propagates in all directions, i.e., toward a side of the n-type cladding layer and a side of the p-type cladding layer. Therefore, in case of the nitride semiconductor light-emitting element in which light having passed through the n-type cladding layer is extracted from a rear side thereof, if part of the light propagating on the side of the p-type cladding layer reaches an interface with the p-electrode and reflected thereby without being absorbed by the p-type contact layer, the reflected light propagates toward the n-type cladding layer and is effectively used. By configuring in such a way that the light propagating toward the side of the p-type cladding layer is reflected and returned to the side of the n-type cladding layer with high efficiency, an amount of light extracted from the nitride semiconductor light-emitting element increases, and therefore the external quantum efficiency of the element is enhanced.
An attempt to improve the external quantum efficiency by efficiently reflecting light propagating on a side of a p-type cladding layer is disclosed in Patent Documents 1 and 2, and Non-Patent Document 3 described below.
According to the technique disclosed in Patent Document 1, a p-electrode to be electrically connected to a p-type contact layer is formed in a mesh pattern having apertures on the p-type contact layer, and a reflective layer using a metal such as silver or Al is formed on the p-type contact layer exposed in the apertures and the p-electrode, so that the external quantum efficiency is improved by providing a structure in which light having passed through a p-type cladding layer and a p-type contact layer is reflected toward the side of the active layer by the reflective layer formed in the apertures.
According to the technique disclosed in Patent Document 2, a high reflectivity metal layer making Ohmic contact with a p-type nitride semiconductor layer and having a mesh pattern with apertures is provided on the p-type nitride semiconductor layer, and further a metal barrier layer for assisting the reflection of the high reflectivity metal layer is provided on the p-type nitride semiconductor layer exposed in the apertures and the high reflectivity metal layer, so that the external quantum efficiency is improved by providing a structure in which light having passed through the p-type nitride semiconductor layer is reflected by an interface between the high reflectivity metal layer and the metal barrier layer.
According to the technique disclosed in Non-Patent Document 3, Pd electrodes of a nano-pixel type making Ohmic contact with a p-type nitride semiconductor layer are provided on the p-type nitride semiconductor layer, and further an Al reflective layer is formed in a gap between the Pd electrodes, so that the external quantum efficiency is improved by providing a structure in which light having passed through the p-type nitride semiconductor layer is reflected toward an active layer by the reflective layer formed in the gap.